This invention relates to dynamoelectric machines, particularly to machines with a superconducting rotor, and to the construction and method for making a warm damper shield for a supercooled rotor.
Superconducting generators have been designed with a supercooled direct current field winding on the rotor. The field winding requires shielding against disturbances that may cause amplitude variations in its direct current that could result in quenching of its superconducting properties. It has been recognized that to achieve these purposes one may use a rotor that includes on its outer or warm surface over the field winding a damper shield that shields against AC fields in the air gap caused by harmonics in the stator and unbalanced loads and also acts as a damper against oscillations due to transients in the stator.
Various aspects of warm damper shields in superconducting rotors are dealt with in Cooper et al U.S. Pat. No. 4,123,676, Oct. 31, 1978, Cooper et al U.S. Pat. No. 4,152,609, May 1, 1979, and copending application Ser. No. 350,527, filed Feb. 19, 1982 by Hooper and assigned to the present assignee, the disclosures of which are herein incorporated by reference. U.S. Pat. No. 4,152,609 has particularly to do with the construction of a warm damper shield consisting of a central cylinder of a conductor such as copper or aluminum with the inner and outer concentric cylinders joined thereon of a high strength, less conductive material, such as one of the superalloys commercially available under the name Inconel. Such a composite cylinder has the ability to provide the necessary electromagnetic and mechanical functions of the warm damper shield and is utilized as part of the present invention.
It is necessary to assemble the structure so that there is a metallurgical bond between the central conductive cylinder and the stronger inner and outer cylinders. Strength is essential as the forces that can be applied to the warm damper assembly due to a possible line fault are about an order of magnitude greater than the centrifugal forces encountered during normal operation of the rotor at 3600 rpm. The requirement for thorough metallurgical bonding has been best achieved by the method of the copending application involving explosive welding of the inner and outer cylinders to the central cylinder. The end portions of the damper shield are a concern however because it is desirable to have high strength material at these regions without the weaker conductive material extending to the extremity of the shield. The ends have to be strong throughout their thickness especially so they can be drilled and tapped to receive bolts fastening them to the end shafts. The required hole size may be greater than the thickness of the central cylinder.
As presented in the copending application there are end cylinders of high strength material axially adjacent the central conductive cylinder and soldered or brazed thereto prior to the explosive welding of the inner and outer support cylinders. While generally feasible, there is need for improved techniques so as to insure against voids and possible crack initiation at the interfaces between the central copper cylinder and the axially adjacent high strength material. While soldering or brazing of the end cylinders to the conductive cylinder is reliable to form the preassembly of those elements, problems can arise upon the explosive welding to that preassembly of the inner and outer support cylinders. Explosive welding inherently produces large forces on the elements that can disturb the metallurgical integrity of the previously formed joints.
These concerns are addressed by the present invention that provides an improved warm damper assembly for a superconducting rotor by a method that involves machining mating grooves in end portions of the conductive core cylinder and the adjacent end portions of higher strength end cylinders and fitting those elements together at the mating grooves prior to the explosive welding of the inner and outer support cylinders. This technique may be performed without solder or brazing material at the groove joint but such material may also be employed if desired for additional security. In a preferred form, the grooves of the mating pieces are screw threads and the end cylinders are assembled to the central cylinder by screwing them on.